102 



THE PROGRESS OF SCIENCE 



proof of the gaseous condition of some of them. 

 Moreover, physicists of the present generation 

 appear now to accept the secular cooling of the 

 earth, which is one of the corollaries of that hy- 

 pothesis. In fact, attempts have been made, by 

 the help of deductions from the data of physics, to 

 lay down an approximate limit to the number of 

 millions of years which have elapsed since the 

 earth was habitable by living beings. If the con- 

 clusions thus reached should stand the test of fur- 

 ther investigation, they will undoubtedly be very 

 valuable. But, whether true or false, they can 

 have no influence upon the doctrine of evolution in 

 its application to living organisms. The occurrence 

 of successive forms of life upon our globe is an 

 historical fact, which cannot be disputed ; and the 

 relation of these successive forms, as stages of evo- 

 lution of the same type, is established in various 

 cases. The biologist has no means of determining 

 the time over which the process of evolution lias 

 extended, but accepts the computation of the 

 physical geologist and the physicist, whatever 

 that may be. 



Evolution, as a philosophical doctrine applicable 

 to all phenomena, whether physical or mental, 

 whether manifested by material atoms or by nu'ii 

 in society, has been dealt with systematically in 

 the "Synthetic Philosophy" of Mr. Herbert 

 Spencer. Comment on that great undertaking 

 would not be in place here. I mention it because, 



